1. Field of the Invention
This invention generally relates to a noise cancelling apparatus for use in a telephone handset and, more particularly, to a noise cancelling apparatus that combines a directional, noise-cancelling microphone structure with a noise-reducing, signal-shaping electronic circuit for reducing ambient noise transmitted through the telephone handset.
2. Description of the Related Art
For most communication systems operating under normal conditions, ambient noise does not pose a serious problem. The noise, as well as the voice signal, is conveyed to the listener without any special processing to eliminate the noise signal. However, in an environment, such as a factory, a computer room, or a construction site, where the ambient noise is quite substantial, the noise element must be taken into consideration. If left unfiltered, the high noise level interferes with and distorts the voice signal, as well as fatigues the listener on the receiving end.
Numerous devices for reducing ambient noise transmission have been contemplated. One such device employs an electronic noise guard circuit that receives an input signal and separates it into two components, a rapidly fluctuating signal and a steady signal. The fluctuating signal is assumed to be the voice signal, while the steady signal is considered to be environmental noise. Thus, the noise guard circuit simply removes the steady component from the input signal, leaving only the voice signal.
One inherent drawback of this method is that it indiscriminately eliminates all steady signals. If the steady component contains not only noise but part of the voice signal as well, then a portion of the voice signal is lost.
Another drawback arises from the oversimplified approach that noise manifests itself as only a steady signal. Clearly, if the ambient noise is fluctuating, the circuit does not suppress the noise.
Another device for suppressing ambient noise involves a voice switching system that compares the input signal to a reference voltage. If the input signal is lower than the reference voltage, the system assumes that the input signal contains only noise and, consequently, suppresses the entire signal. Once the input signal rises above the reference voltage, the circuit allows the entire signal to pass. This arrangement prevents the transmission of noise during the idle portions of a conversation; however, it does not eliminate the background noise when the level of the input signal is above the reference voltage. The reference can be set to a higher level to eliminate more background noise, but in a high noise environment, the reference would have to be set at such a high level that shouting would be necessary.
Some voice switching systems also include a low-pass filter to selectively reduce high frequency noise. These type systems also fail to provide satisfactory results, in that they indiscriminately remove the high-frequency portion of speech, thereby rendering it less understandable.
Yet another device designed to eliminate ambient noise is a directional noise cancelling microphone embodying a diaphragm having at least two ports for receiving audio signals. The diaphragm is designed such that the two noise components of the input audio signals cancel each other, leaving only the voice signal to be transmitted over the telephone handset.
Previous directional noise cancelling microphones suffer from numerous drawbacks, however. They are bulky and cumbersome in design, usually requiring additional components to be attached to the exterior of the telephone handset. Further, the directivity characteristic of these microphones is also poor. In fact, when the position of the microphone deviates slightly from the desired position, the microphone severely distorts the voice of the speaker or even fails to receive the voice signal, resulting in a very undesirable voice cancelling effect. Furthermore, despite the noise cancelling function performed by these microphones, the output signal still contains a substantial noise component.
The present invention is directed to overcoming one or more of the problems set forth above.